The invention relates to an apparatus for decoding a first series of codes representing a first series of differential values corresponding to a row of pixels of a digitised image.
The invention further relates to a method of encoding a first series of values representing a row of pixels of a digitised image.
The invention still further relates to an apparatus for reproducing a stored digitised image, to a method of storing a digitised image and to a storage device wherein an image has been stored in accordance with such a method.
Differential coding for image transmission or storage is well known generally in the art, but a particular example is in the Compact Disc-Interactive system described for example in "CD-I--a Designer's Overview" published by Kluwer (ISBN 9020121103). In the CD-I system, natural photographic images can be encoded and recorded on an optical disc for subsequent retrieval, decoding and display by the CD-I player. The encoding technique used achieves a high degree of data compression, but the data rate of the data channel (the disc reading apparatus) is not high enough to permit sequences of such images to be displayed as moving pictures unless the images are limited to a small area of the available display screen. Storage and retrieval can be generally considered as analogous to transmission and reception, both being characterised by frequency response and noise parameters. The present invention is thus relevant to both transmission and storage systems in which bandwidth is limited.
It is known that such images can be expanded in the known apparatus to fill a larger screen area, but only by repeating the received pixel values to display each value over a large block of pixels. This technique introduces a "mosaic" effect, however, which is visually obtrusive and generally undesirable.
It is further known that satisfactory expansion of digitised images can be achieved by using linear interpolation to generate intermediate pixel values. The interpolation can be performed substantially independently in the line scan (hereinafter "horizontal") and field scan (hereinafter "vertical") directions.
Unfortunately, the differential nature of the coding in the known systems makes known interpolation techniques unsuitable particularly in products such as the CD-I player which are aimed at the consumer market.